Biosynthesis

Biosynthesis and Degradation of Heme

Introduction

Heme is an iron-containing porphyrin compound that serves as a vital prosthetic group in numerous biologically important proteins.
  - Structure:
    - Consists of a protoporphyrin IX ring coordinated with a central ferrous (Fe²⁺) iron atom.
    - Enables participation in reversible redox reactions and oxygen binding.
**Biological Roles: **
  - Central in oxygen transport and storage (as components of hemoglobin and myoglobin, respectively).
  - Essential in various enzymes, including:
    - Cytochromes (involved in electron transport and cellular respiration).
    - Catalase and peroxidase (protect cells against oxidative damage by decomposing hydrogen peroxide).
Heme synthesis and degradation are highly regulated metabolic processes that maintain cellular and systemic homeostasis.
- Synthesis occurs through a multi-step pathway involving mitochondrial and cytosolic enzymes.
- Degradation occurs primarily in macrophages, leading to the formation of biliverdin, bilirubin, and iron.

Heme Structure

Core Components

Porphyrin Ring (Protoporphyrin IX)
- A tetrapyrrole ring formed from four pyrrole rings (labeled A, B, C, D) connected by methine bridges (-CH=).
- Contains side chains of methyl (–CH₃), vinyl (–CH=CH₂), and propionate (–CH₂–CH₂–COO⁻).
Central Metal Ion
   - Iron (Fe²⁺ or Fe³⁺) is chelated at the center.
   - Fe²⁺ (ferrous): Binds oxygen (O₂) in hemoglobin/myoglobin.
   - Fe³⁺ (ferric): Found in cytochromes (used in electron transport).

Biosynthesis of Heme

Site of Synthesis

Heme biosynthesis occurs in two main cellular compartments:
  1. Mitochondria
     - Steps: The first step and the last three steps occur here.
     - Provides:
       - Succinyl-CoA from the TCA cycle.
       - Iron (Fe²⁺) for insertion into the porphyrin ring.
     - The mitochondrial environment is essential for:
       - Rate-limiting regulation of heme formation.
       - Final heme formation.
  2. Cytosol
     - Steps: Intermediate steps occur here where enzymes convert early precursors into tetrapyrrole porphyrin rings.
     - Separation between mitochondria and cytosol allows tight regulation of heme production and prevents accumulation of toxic intermediates.

Steps of Heme Biosynthesis

Condensation of glycine and succinyl-CoA to form $ext{δ-aminolevulinic acid (ALA)}$
- Enzyme: ALA synthase
- Location: Mitochondria
Condensation of two molecules of ALA to form $ext{porphobilinogen (PBG)}$
- Enzyme: ALA dehydratase (PBG synthase)
- Location: Cytosol
Polymerization of four PBG molecules to form $ext{hydroxymethylbilane}$
- Enzyme: Porphobilinogen deaminase
- Location: Cytosol
Cyclization of hydroxymethylbilane to form $ext{uroporphyrinogen III}$
- Enzyme: Uroporphyrinogen III synthase
- Location: Cytosol
Decarboxylation of uroporphyrinogen III to form $ext{coproporphyrinogen III}$
- Enzyme: Uroporphyrinogen decarboxylase
- Location: Cytosol
Oxidation of coproporphyrinogen III to form $ext{protoporphyrinogen IX}$
- Enzyme: Coproporphyrinogen oxidase
- Location: Mitochondria
Oxidation of protoporphyrinogen IX to form $ext{protoporphyrin IX}$
- Enzyme: Protoporphyrinogen oxidase
- Location: Mitochondria
Insertion of ferrous iron (Fe²⁺) into protoporphyrin IX to form heme
- Enzyme: Ferrochelatase
- Location: Mitochondria

Precursors of Heme Biosynthesis

Glycine
- A simple amino acid providing nitrogen and part of the carbon skeleton of the porphyrin ring.
- Enters mitochondria to participate in the first reaction.
Succinyl-CoA
   - Derived from the TCA cycle.
   - Links energy metabolism with heme synthesis.
   - Provides the carbon backbone for $ext{δ-ALA}$ formation.
Vitamin B6 (Pyridoxal Phosphate)
- An essential coenzyme for ALA synthase.

Reactions of Heme Synthesis

Reaction: $ext{Glycine} + ext{Succinyl-CoA} ightarrow ext{δ-Aminolevulinic Acid} + ext{CO₂} + ext{CoA}$
- Enzyme: ALA synthase
- Location: Mitochondria
Reaction: $2 ext{ molecules of ALA} ightarrow ext{Porphobilinogen}$
- Enzyme: ALA dehydratase (PBG synthase)
- Location: Cytosol
Reaction: $4 ext{ molecules of PBG} ightarrow ext{Hydroxymethylbilane}$
- Enzyme: Porphobilinogen deaminase
- Location: Cytosol
Reaction: $ext{Hydroxymethylbilane} ightarrow ext{Uroporphyrinogen III}$
- Enzyme: Uroporphyrinogen III synthase
- Location: Cytosol
Reaction: $ext{Uroporphyrinogen III} ightarrow ext{Coproporphyrinogen III}$
 - Enzyme: Uroporphyrinogen decarboxylase
 - Location: Cytosol
 - Key Features: Coproporphyrinogen III is transported back into mitochondria.
Reaction: Inside the mitochondria, two oxidation reactions occur:
   - Coproporphyrinogen oxidase:
     - Converts coproporphyrinogen III into protoporphyrinogen IX.
   - Protoporphyrinogen oxidase:
     - Converts protoporphyrinogen IX into protoporphyrin IX (direct precursor of heme).
Formation of Heme:
 - Reaction: $ext{Protoporphyrin IX} + ext{Fe}^{2+} ightarrow ext{Heme}$
 - Enzyme: Ferrochelatase
 - Location: Mitochondria
 - Key Features:
 - Inserts ferrous iron (Fe²⁺) into the porphyrin ring to produce functional heme, essential for:
 - Hemoglobin (oxygen transport)
 - Myoglobin (muscle oxygen storage)
 - Cytochromes (electron transport)
 - Step occurs in mitochondria where iron is readily available.

Degradation of Heme

Heme degradation is critical for breaking down heme, an iron-containing porphyrin present mainly in hemoglobin, myoglobin, and cytochromes.
- Red blood cells (RBCs) have a limited lifespan of about 120 days, requiring continuous removal of senescent or damaged RBCs and safe metabolization of their heme component.
This process occurs primarily in the reticuloendothelial system (RES), especially in the macrophages of the spleen, liver, and bone marrow.
During degradation:
  - Heme is converted into bilirubin (a bile pigment), while iron is conserved and recycled for new hemoglobin synthesis.
  - Efficient heme degradation prevents toxic heme accumulation and maintains normal iron and bilirubin homeostasis.
  - Clinically, disturbances in heme degradation or bilirubin metabolism can lead to jaundice and related disorders.

Source of Heme

Heme is primarily derived from hemoglobin released during the destruction of senescent RBCs (≈120-day lifespan).
Minor sources include:
  - Myoglobin
  - Cytochromes
  - Catalase
  - Peroxidases

Conversion of Heme to Biliverdin

Enzyme: Heme Oxygenase (HO)
  - Location: Macrophages
  - Requires: O₂ and NADPH
  - Reaction: Heme → Biliverdin (green pigment)
  - Key Features:
    - The porphyrin ring of heme is opened.
    - Break occurs at the α-methene bridge.
    - This step is the rate-limiting step of heme degradation and is irreversible.
Additional Outcomes:
- Iron (Fe²⁺) is stored as ferritin and hemosiderin, reused for new hemoglobin synthesis.
- Carbon monoxide (CO) is produced, acting as a signaling molecule with vasodilatory and anti-inflammatory roles at low concentrations.

Conversion of Biliverdin to Bilirubin

Enzyme: Biliverdin Reductase
  - Location: Present in macrophages, the same cells breaking down old RBCs.
  - Uses: NADPH or NADH as reducing agent.
  - Reaction: Biliverdin + NADPH (or NADH) → Bilirubin
  - Type: Reduction reaction, adding hydrogen to biliverdin.
Characteristics of Bilirubin:
  - Yellow-orange pigment responsible for the yellow color of bruises and jaundice.
  - Lipid-soluble, cannot circulate freely in blood.
  - Transported tightly bound to albumin (major plasma protein).
  - Cannot be excreted directly in urine; must go to the liver for conjugation.

Conjugation in Liver (Detoxification)

The liver converts unconjugated (lipid-soluble) bilirubin into conjugated (water-soluble) bilirubin, which can be excreted.
Mechanism:
  - Enzyme: UDP-glucuronyl transferase (UGT1A1)
  - Substrate: Bilirubin + UDP-glucuronic acid
  - Reaction: Adds one or two glucuronic acid molecules → forms bilirubin monoglucuronide and diglucuronide (conjugated bilirubin).
Results:
- Conjugated bilirubin is water-soluble (direct bilirubin) and can be secreted into bile.
Key Point:
- Indirect bilirubin = unconjugated (albumin-bound).
- Direct bilirubin = conjugated (water-soluble, excretable).